Want to lose weight? Cut out the ultra-processed foods. A new study finds that such foods, which "have come to dominate the US diet" over the past seven decades, per NPR, have a big impact on weight gain. Researchers at the National Institutes of Health split 20 healthy, stable-weight adults into two groups, and provided all their meals for four weeks. For two weeks, half the study participants were fed only whole foods or minimally processed foods while the other half were fed an ultra-processed diet including foods like canned meat, beans, and fruit; diet lemonade; yogurt; chips; roasted nuts; and jarred condiments. Then the two groups switched. While participants were on the ultra-processed diet, they ate an average of 508 calories more per day than the other group and gained an average of 2 pounds over the two weeks. While eating unprocessed foods, they lost an average of 2 pounds.
The daily menus offered to both groups had the same amounts of calories, fats, protein, sugar, fiber, carbohydrates and salts, but participants were allowed to eat as much as they wanted. Those on the ultra-processed diet chose to consume more than those in the other group—even though the two different types of diets were rated as equally tasty and satisfying, the Los Angeles Times reports. Interestingly, however, both groups consumed the same amount of protein; those on the ultra-processed diet ate more carbs and fat, and they also ate more quickly, possibly because processed food is easier to eat. They also had higher levels of a "hunger hormone" and lower levels of an "appetite-suppressing hormone" than those on the whole foods diet. People tend to assume the high amounts of salt, sugar, and fat in ultra-processed foods drives weight gain, but "when you match the diets for all of those nutrients, something about the ultra-processed foods still drives this big effect on calorie intake," the lead researcher says. (Read more processed foods stories.)